This invention relates to a magnet assembly for adjusting the running paths of the electron beams of a colour picture tube, and more particularly to a magnet assembly for adjusting the running paths of the electron beams including at least one pair of annular shaped magnets surrounding the neck portion of an in-line electron gun type colour picture tube.
Generally, in a colour picture tube provided with in-line electron guns, the axes of three electron guns are arranged on the same plane including the tube axis such that the paths of two outside electron beams emanated from the electron guns located on both sides of the central electron gun will be symmetrical with respect to the path of the electron beam emanated from the central gun. It is advantageous to construct the electron gun assembly of such colour picture tube to cause the electron beams emanating from respective guns but not deflected by deflecting means to converge at the center of a fluorescent screen located in front of the electron gun assembly so as to produce clean pictures.
Actually, however, it is difficult to cause picture tubes to satisfy such ideal condition due to the manufacturing errors of the component parts of the tube or permissible allowance in the dimensions of the component parts. For this reason, the colour picture tubes generally have slightly different characteristics. Accordingly, it is necessary to provide suitable means to correct such error. As is well known in the art adjustable magnetic field has been used to provide the required colour purity adjustment as well as the static convergence adjustment.
For example, the colour purity has been adjusted by mounting a two pole magnet assembly including two superposed two pole annular shaped permanent magnets on the outside of the neck of the colour picture tube so as to surround the running paths of three electron beams and by relatively rotating the two annular shaped permanent magnets.
On the other hand, the static convergence adjustment has been accomplished by providing a pole piece structure for directing the magnetic field on the inside and outside of the neck of the tube and an adjusting magnet combined with the pole piece structure.
It has also been proposed to perform the static convergence adjustment by mounting a four pole magnet assembly including two superposed four pole annular shaped permanent magnets and a six pole magnet assembly including two 6 pole annular shaped permanent magnets on the outside of the neck of the tube to surround the paths of three electron beams. These arrangements are described, for example, in the specifications of U.S. patent application No. 217757 filed on Jan. 14, 1972 by Radio Corporation of America, Japanese patent application No. 58117 of 1972 and Japanese laid open specification No. 82731 of 1973. By adjusting the intensity of the magnetic fields produced by respective magnets of the assembly, the electron beams from respective electron guns are converged at the center of the fluorescent screen under the condition of no deflection.
In terms of the amount of shift of the electron beam on the fluorescent screen, it is necessary to adjust the intensity of the magnetic field of the magnet assembly in a range of from the maximum amount of the shift of the electron beam in which the beam is deviated to the maximum extent from the ideal condition described above and to the minimum amount of the shift of the electron beams. More particularly, where the colour picture type is ideally manufactured so that the electron beams converge at the center of the fluorescent screen with high colour purity, it is not necessary to provide the magnet assembly. Accordingly, the magnet assembly should be manufactured such that even when the magnet assembly is mounted upon the tube, the magnet assembly will never affect the electron beams. In this case, it is not necessary to vary the intensity of the magnetic field produced by the magnet assembly.
In order to vary the intensity of the magnetic field of the magnet assembly, it is usual to constitute the magnet assembly by superposing two annular shaped permanent magnets having the same polarity and the same flux density. Where the polarities of the two permanent magnets are aligned the intensity of the field of the magnet assembly becomes maximum. On the other hand, where the two permanent magnets are rotated relatively over a predetermined angles, for example 180.degree., 90.degree. or 60.degree., the intensity of the field of the magnet assembly becomes munimum.
With this construction, however, a magnet assembly capable of shifting the electron beams to the maximum extent is easily obtained by increasing the flux density of the magnets whereas it is difficult to manufacture a magnet assembly whose amount of shifting the electron beam is minimum or substantially zero. Accordingly, where such magnet assembly is mounted on a substantially ideal colour picture tube, the minimum amount of shift of the electron beam is too large thus making it impossible to obtain excellent convergence adjustment.